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February 29, 2008

HMT Forecast Discussion 1945z F 29 Feb 08

Overall outlook for significant pcpn in the ARB continues bleak. Low center is heading for the Queen Charlottes at present, and frontal band of middle and high clouds intersects the Coast near CA-OR border, and thence southwest to west of KSFO. Prospects are for possibility of some light pcpn tonite in the upper elevations of the ARB, but air mass is mainly dry and the frontal band may well come across trailing only virga.

Behind this first system, high pressure is building smartly and will move into the Northwest on Saturday 1 March. The next upstream system is forecast to stay well to the N as it crosses the Coast late M - Tu 3 and 4 March. Indications are that this system will then dig swd and quite likely cutoff over the Southwest or Sonora. This morning's hi-res GFS run is not as serious about this as are many of the GFS ensemble members or the ECMWF: many of the ensemble members as well as the ECMWF build the ridge more strongly just off the Coast early next week, both creating a strong Nly flow down the Coast and effectively blocking future upstream systems from affecting California. As a consequence, the 12z today GFS hi-res is an outlier in its setting up a strong zonal flow across the central and eastern Pacific by M 10 March with significant pcpn into the ARB. Partly for this reason, and also partly because the GFS often is too conservative on West Coast digging trough scenarios that lead to cutoff low formation, consensus is that we are not going to see any significant pcpn in the ARB till at least 10 March. For reference, the Penn State E-wall web site GFS ensembles from the 00z 29 Feb runs (last night) keep the ARB 100% dry (after tonite's whatever) through 12z F 7 March, and the 12z ensembles are all dry through 12z M 10 March except the aforementioned hi res GFS.

John B.

February 28, 2008

HMT Forecast Discussion 1945Z Th 28 Feb 08

Overall conditions appear unfavorable for IOP candidates for the next week or more. The best opportunity at present is from full-latitude mobile Pacific trough near 150W. This feature is forecast to move steadily toward the coast over the next 48h, with a surface low developing and moving newd toward the Queen Charlottes by late Friday the 29th. The trough aloft crosses the HMT late Friday night and Saturday morning. Unfortunately, there is no low-level WAA with this complex, and the sea-level geostrophic wind becomes difluent as the associated sfc frontal system approaches the coast. What this implies is that the system will be relatively dry at and south of the latitude of the HMT, though topographic forcing for pcpn on the higher terrain will be favorable. I don't see any reason to depart significantly from the GFS and EC on overall trends with this system, but I think up to 0.75" pcpn is possible at higher elevations in the ARB, with max pcpn 0.5" more likely. Heavier pcpn will fall on the Siskiyous and northward.

Looking farther ahead, models indicate ridge buliding smartly into the Pacific NW and a resumption of troughing over the central US. All this bodes poorly for any IOP potential for the next 7 days or so.

John B.

February 27, 2008

HMT Forecast Discussion for Wednesday 27 Feb 08 (entered 1950z)

The Pacific continues to be quite active, but as discussed yesterday a mean ridge along or near the West Coast could keep the ARB out of action for quite a while. Two strong systems currently in the Pacific, one near 45N/150E, and the other near 45N/150W, with this one stretching from the Gulf of Alaska way back to the sw with a nice tap into tropical moisture. The main part of the strong system that was east of this wave is now hitting far northern British Columbia with a weakening trough stretching back to off the CA coast, but this continues to weaken and will not produce any precip in the ARB tomorrow. So the next potential system is the strong eastern Pacific one noted above, but this system will also head to the northeast with the main impact in the Pacific Northwest late Friday and Saturday. This leaves the ARB at the extreme southern end of the system. The deterministic 00z runs were overall similar to what we saw yesterday except maybe a tad farther north with the wave on Sat/1 March. As a result the 00z/GFS has only a trace of precip near KBLU on Saturday, while the ECMWF has less as well, with maybe a tenth of an inch or so. The 00z/Canadian Global run has a little more, maybe 0.25" in the ARB higher elevations, and the NOGAPS around 0.2 or so. All the models have more of course not too far away to the north, but all also keep this as a fast-moving wave. The 00z ensembles are also favoring a near miss, though there are a few GFS members with more precip than the deterministic run, and this is even more the case for the Canadian Global ensemble members. The 09z/SREF is like the larger scale ensembles, most do not have much but a few members have up to 0.25" or so. The 12z models coming in paint a similar picture; the 12z/NAM produces only ~0.10" at KBLU, with a similar amount from the 12z/GFS, and maybe a slight bit more from the 12z/Canadian Global model, while the 12z/ECMWF just in has only 0.01" at KBLU. So this remains a close call but not a system to be IOP-worthy, unless things change considerably.

The next wave, a much weakened version of the system now in the western Pacific, is forecast to pass mostly to the north and over the upper level ridge, barely brushing the ARB on Mon/3 Mar. There are only a few of the 00z Canadian ensemble members that bring any precip to the ARB with this system, but quite a bit does fall over the Pacific Northwest where the remnants of its tropical connection make the most impact. The 12z/ECMWF forecasts almost a tenth of an inch at KBLU, but most other models have nothing for the ARB. Overall there continues to be general agreement in the 00z deterministic runs and in the ensembles that this wave dives into the CONUS resulting in a longwave trough by mid next week, with an upper level ridge to its west along the West Coast that then keeps things dry through most of the 10-day forecast period ending on 8 March.

Forecasts for days 10-15 ending Thu/13 Mar, available from the Global Canadian and GFS and their ensembles, continue to show some potential for action in the ARB with a long fetch of more zonal flow across the Pacific breaking down the West Coast ridge. But overall the 00z ensembles are slightly less favorable than those from yesterday in agreeing on a big change, with a number of members, especially from the GFS, keeping a ridge that is retrograded but still close enough to the West Coast to keep things dry in the ARB. Basically by 8 Mar a couple of Canadian ensemble members have some precip, and then the number of members grows each day farther out, including a few from the GFS, though at no day do the majority of members support a more zonal flow into the West Coast. This is seen nicely in the NAEFS meteogram presentations, which for KSAC show very little precip for the median of the ensembles, but a trend towards greater potential from a few of the members, beginning on 7 March. Moving up the coast to Portland, the same presentation has greater median precip each day from 7-13 March, as well of course as for the next two storms that are forecast to pass to the north of the ARB. This indicates the active nature of the Pacific, which means that a southward shift to the storm track would put the ARB back in action by the second week in March, but that such a shift currently does not have a high probability of occurring, according to the 00z ensembles. A look at the 12z deterministic GFS provides more optimism for the 8-14 Mar period, with a series of waves chrashing the coast much farther to the south than shown by the 00z GFS. The 12z/ECMWF takes a few days longer before it looks like things could break down, so that by the last day of the ECMWF forecast (day 10, 12z/Sat/8 Mar) the ECMWF is still dry in the ARB but has a potent system approaching, while the 12z/GFS already has a sharp wave about to produce precip in the ARB. The 12z/GFS and Global Canadian ensembles also have more members now with potential systems hitting the ARB for the 8 to 13 Mar period, moreso than was seen in last night's 00z ensemble set. Not sure why the 12z runs should necessarily be more favorable-looking than the 00z runs, but basically this supports the notion that things could get more interesting again as early as 8 March and certainly by the second week in March for the ARB.

ed szoke NOAA/GSD/CIRA

February 26, 2008

HMT Forecast Discussion for Tuesday 26 Feb 08 (entered 2000z)

As noted yesterday, quite active across the Pacific but an upper level ridge has built along the West Coast behind the weekend storm and this is forecast to be a long-lived feature that could keep the Pacific systems north of the ARB for an extended period. The next system in line is the big one that built the upstream ridge for our weekend storm. It is now splitting with the main energy heading far north while the much weaker southern portion separates and drifts eastward. Currently the southern portion is near 35N/142W with a nice tap into moisture east of Hawaii. However, the system is quite weak when it comes onshore late tomorrow with little of this moisture remaining. Another strong system is behind this and also has a nice tap into tropical moisture, and it does squash the ridge some as it hits late Friday into Saturday, but most of the energy rides to the north. It is a close call and could see some precip into the ARB on Sat/1 Mar, but both the 00z ECMWF and 00z/GFS and in fact all the other 00z models keep most if not almost all of the precip north of the ARB. The latest 12z GFS is maybe a tad farther south and predicts ~0.25" at KBLU on Sat/1 Mar. The 12z/ECMWF just in is also a little further south with up to 0.30" at KBLU and almost 0.40" over higher terrain to the east on Saturday. On the same vein, the 12z/Canadian Global model also is a tad further south with ~0.3" of precip in the higher elevations of the ARB. It is a close call on a good number of the 00z ensemble members, with a few of the Canadian ensemble members and a couple of the GFS members bringing more substantial precip into the ARB, but still with a fast-moving system. Some of the 12z/GFS ensemble members also have some precip now sinking south into the ARB, but all keep the maximum to the north more over far Northern California and in OR/WA. It would take a farther south track to probably make this storm anything of interest, and even then it is still a fast-moving system. The one thing to keep in mind though is the current strong connection to tropical moisture stretching currently all the way back to the sw Pacific.

The next wave is not as strong as the Saturday system and there is good agreement that the precip with this open wave stays north of the ARB on Tue/4 Mar. This wave then drops into the central CONUS and phases with energy rotating around a cold Hudsons Bay vortex to form a big trough over the mid-CONUS by the middle of next week, with a high-amplitude ridge building along the West Coast behind this system. There continues to be very good agreement in the 00z ensembles on this evolution in both the Canadian and GFS ensemble systems, as well as in the 12z/GFS ensembles. As discussed yesterday, the Pacific remains quite active through early March and some of this energy finally breaks into CA by the second week in March. This is seen in a number of the 00z Canadian ensemble members with the first potential event on Sat/8 Mar and then continuing till the end of the forecast (12-13 Mar). The 00z GFS ensembles are largely dry although the deterministic run of the 00z/GFS does bring some precip into the ARB by Wed/12 Mar. The 12z/GFS deterministic run is more aggressive and has the first chance of precip by 8 Mar followed by other potential events through the end of the run on 13 Mar, and the 12z/GFS ensemble also have several members with periods of precip beginning near 8 Mar or during the next week. So at this point it still looks like a potentially active week by the second week in March, but baring some turn of events, probably not much before then.

ed szoke NOAA/GSD/CIRA

February 25, 2008

HMT Forecast Discussion for Monday 25 Feb 08 (issued 1740z/Monday)

The storm has come to an end with most of the precip winding down around
04z to 06z but some lingering higher elevation snows finally ending at
KBLU around 12z this morning. Quite a lot of precip yesterday even
though the surface low and upper level system were weakening as the
storm came onshore in far nw CA during the afternoon. Some totals for
the event from the ESRL gages include: Alta, ~2.6" and Big Bend ~4.3";
the BLU precip gage was down for Sunday but the snow gage was working
and it looks like a storm total of near 20" or so. The closest ski area to
KBLU reported almost 4 feet of snow according to WFO SAC.
CNRFC data has storm totals of 2.86" at KBLU (1.05" and
1.81" in the two 24-h periods ending
at 12z/24 Feb and 12z/25 Feb), 0.80" at KSAC (0.24" and 0.56"
respectively), and 2.76" at Huysink (1.32" and 1.44" respectively). It
snowed heavy at times at KBLU on Sunday but around the time of the
trough passage in the evening the temperature at the KBLU METAR warmed
to 34F along with a supposed changeover to some light rain and mixed
precip from 03z to 05z/Mon. This time it could be real given the warmer
temperatures, as opposed to the mixed precip reports from the KBLU METAR
late Saturday night that were pretty much discounted in yesterday's
telcon as being heavier and wetter snow falling. WFO SAC forecasters
were not sure about the changeover this time, since the temps were warmer
it was feasible. Winds were brisk but not excessive yesterday at KBLU,
peak gusts from the S to SSE of 33-34 kts from 21-23z, matching the
strongest winds that occurred earlier in the storm between 03-10z/Sun.
Winds gradually decreased later in the
day with a shift to a 200-210 wind in the evening with the warmup and
trough passage, then things pretty much calmed down. So with the big
occlusion there was not a dramatic frontal passage like we saw with the
more concentrated early January IOP, but certainly a good system with
some interesting complexities, especially in regards to rain vs. snow at
the radar site (a close call at times, and a tough forecast for sure).

Onto the current weather, the Pacific remains active and this will be
the case over the next 2 weeks. Despite this the ARB may get shut out
from much action as the systems weaken as they approach a mean ridge
forecast to be near the West Coast, though with all the activity in the
Pacific there is uncertainty as we go out farther in the forecast.
Right now the weakening system that was a very strong wave in the
mid-Pacific is a north-south trough near 150W with a nice tap to
tropical moisture, but forecast to continue to weaken and split, with
the main portion riding north of CA where a ridge is now in place, and
another piece becoming a very weak cutoff that approaches the West Coast
midweek but with diminishing moisture. West of this system is another
trough near 165E with a nice tap back to the sw all the way to the sw
Pacific. Behind this is another system about to emerge off of Asia.
The system currently near 160E is the one that brings a threat of
something by Sat/1 March, but even the more favorable models, such as
the 00z/ECMWF and 00z/Global Canadian, have this as a fast-moving event
with the heaviest precip to the north of the ARB, so probably not
IOP-worthy, but certainly worth watching this week given its current
nice moisture tap into a tropical source. The GFS ensembles from 00z
have good agreement in keeping this system north of the ARB, but there
are some members of the Canadian ensemble system that do bring some
precip into the ARB, though even these have a fast-moving event.

The next in the line that will come off of eastern Asia is predicted to
wrap up into the Gulf of Alaska later this week with most of the energy
again staying to the north. Timing and strength differences show up
with this one for very early next week but at this time this system also
is forecast by the deterministic runs to be too weak and fast-moving.
Ensembles generally agree, again, a few are close to bringing something
to the ARB. The models generally forecast this wave to then dive into
the Intermountain West and amplify midweek of the first week of March,
putting an amplifying ridge along the West Coast. This solution has
pretty broad support in the 00z ensemble forecasts. However, a lot of
energy is forecast by this time period to be in the mid to western
Pacific, and the latest deterministic 12z/GFS finally pushes some of
this into the West Coast around 10-11 March, as do some members of the
00z ensembles. Right now the ensembles would be pointing to the best
potential for an active ARB for the second week in March, with some
close calls possible in the first week.

ed szoke NOAA/GSD/CIRA

February 24, 2008

HMT forecast discussion for Sunday 24 February 2008 (entered at 2000z)

The cooler overnight temperatures from Friday night and drying at KBLU mentioned in yesterday's telcon by WFO SAC led to cooling as the precip began yesterday around 21z that was apparently too much to overcome last night, despite a valiant effort by the warm advection, so it was mainly snow at KBLU. We discussed this in the telcon today and WFO SAC noted that between 21z/Sat-00z/Sun they were getting spotter reports of snow down to nearly 3000 ft. They noted though that this was collaborated with the Sloughhouse snow level data for this time, and they made a point today in noting how useful that data was to adjusting their forecast yesterday. Special soundings at Sloughhouse (SHS) on Saturday afternoon did show the warm advection that occurred by 21z and beyond, and there is a sharp temperature rise in the Alta time series last evening from 02-06z (from 0.9C up to 3.6C). A similar timing for the temperature rise in the BLU time series but never enough to get it above freezing overnight, with the gage snowfall showing 10-12" so far and ~1.4" of precip (tho there is a problem noted during the telcon with this sensor, stopping this morning at 1.4". As of 20z, Big Bend has received nearly 3" according to their hot plate gage. The KBLU METAR, if it is to be believed on precip type, did have a changeover to freezing rain and sleet between 07-09z, with the temp near 32F, but now (16z) it is back to 30 with heavy snow. Discussion of this in today's telcon by WFO SAC suggests that rather than UP/FZRA this is probably wetter snowflakes that fall faster, fooling the sensor, but showing that the precip during this time was as close as it got to any changeover to rain (the forecasters at SAC estimate the snow level may have risen during the 02-06z period as high as 5200 ft for a time. Peak gusts overnight from the KBLU METAR were 36 kt around 05z. The latest radar loop picks up the center of the occluded surface low approaching the far northern CA coast and heading for landfall ~21z near the CA/OR border, with a large area of precip over the ARB, including a nice band approximately north to south across KSAC area.

The 12z/NAM analysis has the upper low at 534 dm with the surface low at ~986 mb, and the rapid filling is forecast to continue, with an open wave trough passage at KBLU at 500 mb near 00z/Mon, maybe a few hours later at 700 mb, with all the energy then shifting east of the Rockies where a new storm will be born east of CO tonight. Substantial precip will continue today before ending near 06z/Mon, with the 12z/NAM forecasting up to an additional 2" over the higher terrain of the ARB. This timing is similar to our hi-res models from 12z, which show a rapid decrease in echo coverage after 06z and pretty much everything gone by 11z/Mon, and accumulations up to 2-3 inches and a little higher in some areas of the ARB (in the individual runs; ensemble forecasts from 12z are in the 1.5-2 inch range). At this point the precip should remain as snow at KBLU, but discussion of the soundings is next.

A time series of the OAK RAOBs overnight indicated warming at low levels but cooling aloft with a very unstable lower level profile by 09z, but now the 12z RAOB is more moist adiabatic with quite a bit of warming between 850-700 mb, and a freezing level rising to 5650 ft/822 mb. RNO soundings have been cooling aloft all night but in the 12z sounding very low level warming putting the freezing level to almost 6000 ft. The latest (12z) SHS sounding has the freezing level near 837 mb/1570 m, just a little higher than the 09z sounding. Time series of the winds from SHS shows a little decrease in speed but continued south to sw flow a about 40kts through 4 km MSL. A look at forecast soundings from the RUC 15z run interpolated to KBLU indicates not enough warming to change the precip over to rain today, with just a little very low level warming but the upper levels remaining about where they are. Trough passage at 500 mb is also forecast to be near 00z/Mon, similar to the NAM. The 3-km 12z/Schultz microphysics run does show the freezing level rising this afternoon near KBLU to almost 800 mb, but the precip remaining as frozen down to KBLU, so some shallow warming as also shown in the RUC forecast soundings.

As for the longer range...the strong but weakening system west of our current storm is now splitting in the NAM and GFS forecasts, but what ends up coming across the ARB midweek is a feeble-looking upper low with not much moisture left in it, even though right now this system has a great tap to a tropical plume stretching south to Hawaii. But none of the models has any precip with this when the weak southern part of the system drifts towards CA on Wed/27 Feb. The next threat is then late Friday into Sat, but the 12z GFS remains consistent in keeping this system north of the ARB and fast-moving. In fact, the deterministic run of the 12z GFS keeps it dry in the ARB through 11 March with enough ridging along the West Coast to keep the active Pacific systems north of the ARB. Some variation though in the ensemble members longer range in the GFS, as well as in the 00z Canadian ensemble system. Some members are also more favorable for the Saturday system, including the 00z/ECMWF and Canadian Global runs, so will have to watch this one as the week unfolds, though right now even if it is far enough south it looks to be very progressive.

ed szoke NOAA/CIRA/GSD

February 23, 2008

HMT forecast discussion for Saturday 23 February 2008 (entered 2020z)

Wrapped up surface low analyzed at ~974 mb (528 dm at 500 mb) in the 12z/NAM and mb in the 12z/GFS is sitting west of KSFO this morning with initial bands of precip already on the coast west of the ARB. Not a strong elongated plume into this system but a tap into the plume that does extend well w-wsw into the Pacific. The intensity of the storm will make the most of whatever tap it is making, certainly though not a classic-looking elongated atmospheric river into the ARB, as there is a very strong system to the west near 160W (the one that built the upstream ridge for our current storm) that is also tapping into the stream of moisture in the Pacific. The measurements of IPW this morning do show a rapid increase as the system approaches, with Bodega Bay increasing in the last ~6h from 0.25" to now close to 0.8" from the GPS site there. The upward trend has just begun at the Big Bend site. The 12z OAK sounding still has relatively light southerly flow with a freezing level near 850 mb. The model analyses indicate however that much stronger winds were just off the coast at 12z this morning and the flow is forecast to rapidly increase out of the sw today. This is in fact being seen by the various ESRL profilers, with the Bodega Bay profiler now (19z) showing 50+ kt flow from near 2 km and higher out of the south (more sw by 3.5 km MSL). Sloughhouse winds have also dramatically increased in the last few hours. The NAM predicts 700 mb winds near KBLU approaching 50 kts by 00z/Sun then peaking around 65 kts overnight. Winds continue out of the sw tomorrow and will probably remain quite strong (~50 kt flow at 700 mb for most of the day) then gradually weaken very late as the concentrated upper low weakens rapidly after 12z/Sun, to an open wave at 700/500 mb by 00z/Mon. The winds shift at 700 mb to more westerly after 00z/Mon with the trough passage at 700 mb; maybe a few hours earlier with a shift to more westerly flow at 500 mb. This scenario is forecast in good agreement with both the NAM and the GFS from this morning. WFO SAC indicated strongest winds at KBLU likely to be from about 06z/Sun into Sunday afternoon.

In terms of the timing of the precip, still pretty much as discussed yesterday. The larger scale models indicate precip beginning by 21z in the ARB. Our local models have the first main band into the ARB at 21z this afternoon, then heavy precip continuing through 00z/Mon, then gradually becoming more terrain forced and slowly decreasing, with most over by 06z and totally done by 12z/Mon. This is similar to the 12z NAM and GFS, which have heavy precip through ~06z/Mon, then over by 12z/Mon. Totals from the NAM are max amounts in the ARB near 4", with 2-2.5" in the GFS. A large area in the 4-5" range from our 3 km individual runs with embedded spots above 5". Our ensemble mean totals from the 06z runs gives ~3.5" or so max in the ARB for the event. Discussion during the telcon noted that right now the first band moving into the higher terrain of the ARB is hitting some drier air and widespread virga is occurring but no precip yet (as of 20z). So maybe more like 22z or so for the start of precip at KBLU.

The precip type forecast continues to be tricky, with the current temperature at KBLU at 32F at 20z. However, as the winds increase out of the sw a slug of warmer air is forecast to push northward and it looks likely that the freezing level will rise to at least 800 mb/~6000 ft or so by later this afternoon and remain above KBLU into the evening. There was considerable discussion on this aspect of the storm. WFO SAC noted that with some clearing last night there was more cooling and drying in the higher terrain, and the stronger winds and warming aloft are still a bit off to the west, so actually with wet-bulb cooling as the atmosphere initially saturates the precip should start as snow at KBLU this afternoon. Then it will probably (tho not a certainty) change over to rain for at least a few hours before further dynamic cooling brings it back to snow, probably near or after 06z. This is also seen in the NAM and GFS, with gradual cooling as the upper low approaches but and it still looks like the precip at KBLU would go over to snow around 06z/Sun. However, this is also somewhat uncertain; our local model keeps the freezing level close to 800 mb in the vicinity of KBLU through the entire day on Sunday, having only gradually come down from reaching closer to 750 mb later today. It seems unlikely that it would stay warm enough for all rain past 06z or 12z/Sun, but the snow conceivably could be so wet it might slip off the radar (or hopelessly stick to it driven by the strong winds?). WFO SAC also noted on the BUFR soundings (from the NAM) that there was a little bit of warming aloft actually tomorrow just ahead of the Pacific cold front, so there was some talk of maybe precip at times going back to rain, tho this seemed unlikely in the view of WFO RNO. Our latest 18z runs from our local models show a freezing level right near or even above KBLU into Sunday, but precip type as frozen down to just below KBLU. This is certainly a very tricky part to the forecast. We decided at the telcon to revisit this with another call at 430pm between John and I, and then later John and Dave would talk. At issue is what the radar crew would do in terms of leaving the site as the precip goes to snow and radar operations become marginal.

A brief look at the longer range...this week still should be dry with the next wave riding north into the Pacific NW; this storm is the strong one now near 160W which has a great tap of Pacific moisture, but weakens dramatically over the next few days. However, some models (EC, Canadian, NOGAPS) split this system and move a very weak closed low towards the nrn/ctl CA coast ~Thu/28 Feb, probably not enough to be an event but may bear watching. It still looks like the next potential system of note could be late Friday into the weekend, although the latest 12z GFS is not favorable being fast-moving and a bit north. Other models and some ensemble members are farther south with this system, however, and the GFS PW look from the U of Hawaii does show at least some Pacific connection intact as it approaches CA. Beyond that into the first week of March the deterministic 12z GFS run keeps a ridge along the West Coast, but other models/ensemble members remain interesting with an active Pacific and progressive systems that do reach the coast, so quite a bit of uncertainty in early March at this time.

ed szoke NOAA/CIRA/GSD

February 22, 2008

HMT Forecast Discussion for Friday 22 Feb 08 (entered 2030z)

Bands of precip still coming thru the ARB as with weak upper low and surface low now lifting north off the WA/OR coast, with ill-defined 500 mb trough axis extending to the southeast. More westerly flow now in
place from the ARB westward at 500 mb and within this flow are still acouple of small-scale shortwaves, the last of which is still approaching the nw CA coast. Flow still southwesterly at 700 mb down to the surface
with the temperature at KBLU hovering around 32. Some precip totals so far include, as of 15z, ~1.4" at Alta, about the same at BLU (update as of 20z is close to 1.6" at BLU) but the ESRL gage showing ~10" of snow. The precip should continue until the final shortwave passes a little after 00z/Sat, with the NAM having about another 0.5" at KBLU. A look at the 12z run of one of our hi-res models shows the echoes starting to end by 00z and pretty much totally over by 03z/Sat, but still with local accumulations of 1.5-2" in the forecast.
The ensemble mean was not in yet; probably additional precip will be somewhere between the NAM and
the hi-res forecast for today, maybe another inch or so at most but probably less, in the form of snow at KBLU, with additional cooling beginning later today.

The big weekend storm is starting to take shape in the Pacific. Satellite imagery showing rapid enhancement near 35N/150W over the last 6 h (thru 12z this morning) and continuing now through 18z as the ridge to its west amplifies ahead of the very strong wave that has now moved east to 40N/170E. A look at the
various models indicates not a lot of change from what we were thinking during yesterday's telcon. Maybe a tad slower with when the system moves onshore, now consensus being ~15z on Sunday, with the GFS and ECMWF now in quite good agreement on moving the weakening system into northern CA and then as an open wave eastward. The forecasts then would have an open wave and trough axis passing the ARB around 21z/Sun to 00z/Mon. Here are some preip totals for the storm: 00z/ECMWF, 2.2" at KBLU
(more to the southeast) with 1.3" at KSAC; 12z/ECMWF, 1.8" at KBLU and 1.2" at KSAC; 12z/NAM, 3.75" at KBLU and 1.2" at KSAC; 12z/GFS, 2.9" at KBLU with 1.1" at KSAC. Precipitation looks to begin on Saturday afternoon, likely by 21z. Our hi-res runs increase the echoes by 22z/Sat and then things really pick up after
00z/Sun. Strong period of precip then overnight on Saturday night but then again on Sunday as the upper low gets closer, with totals by late Sunday in the 4-5 inch range. There will be some rapid warming as the sw flow increases to 50-60+kts from 850 mb through 700 mb by late Saturday into early Sunday. Consensus on the telcon is there should be strong warm advection Saturday afternoon and this may be enough to have the precip begin as rain (or snow over to rain) at KBLU, with the snow level rising to maybe 6000 ft or so. As noted yesterday a tricky call for this, if it is rain probably 6 h or so at most and then transitioning to snow by around 06z/Sun. The NAM BUFR soundings never really get much above freezing at KBLU, but the GFS model soundings by 00z/Sun bring the freezing level up to near 800 mb and then lower it towards 850 mb by 06z and then certainly by 12z/Sun. Our local model shows strong warming late Saturday afternoon with the freezing level rising to ~800 mb then gradually lowering after ~06z/Sun. So this would support a period of rain at the beginning of the event. Then heavy snows through Sunday with gradual cooling as the trough
approaches. With the weakening of the trough and the remnants of it passing well to the nw of the ARB probably not anywhere near as dramatic a trough passage as in the early Jan IOP, instead winds by later Sunday shifting to more westerly and the snow at KBLU gradually coming to an end overnight Sunday night, with increasingly lighter density snows as Sunday goes on.

As for the longer range, still looks dry next week as upper level ridging moves over the area. But the next wave now in both the ECMWF and latest GFS for ~Sat/1 March timeframe, with a growing consensus of something possible by then in the ensembles. After that for the following first week of March quite a bit of spread in the ensembles at this point.

ed szoke NOAA/GSD/CIRA

February 21, 2008

HMT forecast discussion for Thursday 21 February 2008 (entered 2045z)

Relatively disorganized system still looking to come across the ARB in two stages, as discussed in yesterday's telcon. First part has entered the lower elevations as a weakening ~north-south band with precip at KBLU beginning as snow at 18z with a temperature of 34, now (20z) down to 32 with snow. A couple of other bands follow the first one, and the weakening of the first band is consistent with the latest 12z NAM, which has the main precip beginning about 18z today. A surface low sits off the far northern CA coast, with a trailing cold front southward that is also still off the coast at 20z, with evidence of a couple of weak shortwaves in the more westerly flow behind the front. There is an extensive moisture plume stretching back from this system to the west-southwest all the way back to the sw Pacific. Satellite PW values close to 1.5-2" in the eastern portion of the plume, much higher well west of the Dateline. The moisture plume is aligned near and just south of an upper level jet with max speeds near 150 kt stretching back to the Dateline. Another even stronger jet extends off Asia across Japan with the speed max south of a strong wave near 40N/160E. This is not the strong wave that will hit the West Coast this weekend, but instead the one that rapidly amplifies an upper level ridge near the Dateline, with a downstream amplification of a shortwave now near 40N/165W that is the next system. This shortwave intensifies remarkably from its current status to become an intense upper level small-scale low, down to 525 dm by 12z Sat near 35N/135W in the 00z/EC, which has now come on board with a very strong wave; in fact, the new ECMWF is even stronger, bottoming out the surface low at 968 mb at 12z Sat and the upper low at 524 dm, lower than the 12z GFS which has 977 mb at this time. This strong storm is forecast to track into northern CA later on Sunday and weaken rapidly. As it approaches strengthening sw flow ahead of the upper level wave should bring warming to the higher elevations of the ARB, with the threat of rain for at least the start of the storm at KBLU. More on that later, but first details on today/Friday's event.

In the latest NAM the increase in precip begins after 18z today, which is in fact happening, with precip continuing through ~21z/Fri (with a total for this period of ~1.5" at KBLU but only ~0.35" at KSAC) with a trough passage at 500 mb around 08z/Fri, then this is followed by another weaker shortwave that increases the precip late Friday for about a 6-h period decreasing around 06z/Sat (for an additional ~0.4" at KBLU and ~0.10" at KSAC), but with the precip falling with colder temperatures. Somewhat uncertain how much will occur with this second bit, may not be of great interest to the HMT, but can be addressed in tomorrow's telcon. Overall storm totals in the higher terrain of the ARB then by Saturday morning (precip ending by 09z/Sat) in the 12z NAM range from 2-2.5". It is currently (16z) right at 32F at KBLU and precip will be all snow with this storm, with temperatures holding steady today then falling tonight with the passage of the lead wave. In the 00z/ECMWF run total precip from this system is on the order of an inch at KBLU and 0.70" at KSAC, with generally similar timing to the NAM. The latest 12z/GFS predicts 1.1" at KBLU through 18z/Fri with 0.35" at KSAC, then another burst like in the NGM later Friday through 12z/Sat but with only ~0.25" more at both KBLU and KSAC. What I've seen of the local models is similar to the NAM, but with maybe a tad more max precip. The 09z SREF mean precip through the event is around 1.5", with some of the Eta members being more moist.

The more interesting and stronger storm continues to be the next one for Sat into Mon, an intense storm that will tap into the moisture plume and send strong sw flow into the ARB. The starting time for the main precip with this storm is late Saturday (~23z in the 12z/NAM and similar in the 00z/ECMWF, but maybe a bit earlier in the latest GFS) with heavy precip overnight Saturday through the day on Sunday. This is a little faster than the NAM but similar to the 00z/ECMWF run. Some precip totals for this storm include: for the 00z/ECMWF 2.2" at KBLU and 1.2" at KSAC (through the end of precip around 12z/Mon/25 Feb); in the 12z/NAM about 3" at KBLU and 0.7" at KSAC (this is through the end of the run at 00z/Mon, so more would be expected for the entire storm); in the latest GFS about 4.2" at KBLU and 1.5" at KSAC; about 2-3" over the higher terrain (more to the south of the ARB) in the 12z/Canadian Global run, with similar timing to the EC, and about 2-3" total in the 12z/NOGAPS, but with a start time a bit later, around 00z/Sun, and finally the 09z SREF mean precip is in the order of 2.5" (through 00z/Mon). So good agreement on a significant event for the ARB. Considerable discussion of course during the telcon with lots of great input from various forecasters. Generally felt that max amounts of up to 4" likely with this event, maybe more given the uncertainties in the amount of moisture in the plume off the coast and the exact track of the wave, plus how intense it may be when it hits the coast. In this regard, the models generally agree that a strong surface low will be approaching the northern Calfornia coast by 12z/Sun, with the GFS at 985 mb and the EC 984 mb. But the EC is about 175 miles further south that the GFS, and this trend will have to be watched. Rapid weakening is forecast over the next 12-h period, with the position differences increasing so that by 00z/Mon the GFS has a 1003 mb surface low in nw OR, while the ECMWF has a more elongated low with the lowest pressure of 1004 mb in northern CA. Both scenarios will bring very strong sw winds into the ARB on Saturday into Sunday, with WFO SAC about to hoist a High Wind Watch for the area.

In terms of the snow level, it looks like it will be a tricky call, at least at storm onset and in the initial phases of the event, in terms of rain vs. snow at the radar site (BLU). This was also extensively discussed in the telcon, with possibility raised of a 6-h or so initial period of rain as warm air worked northwards on Saturday. WFO SAC favored all snow for the event, but other opinions ranged to an initial snow level of around 6000 feet. Some differences on starting time for the precip, but likely to be Saturday afternoon, with the heaviest 12-h period 00z-12z/Sun. A look at the BUFR soundings for KBLU from the 12z NAM indicates increasing deep sw flow, reaching 50+ kts by Saturday afternoon, but temperatures still warming only to near freezing at the surface and cooler aloft, with the entire saturated profile remaining near or below freezing through 00z/Mon. No BUFR soundings available (to me) from the GFS, so examined the model interpolated soundings only that have lower resolution. These soundings for KBLU have more initial warming, with a freezing level near 760 mb at 00z/Sun, which then quickly lowers to near 800 mb by 06z and then to near the KBLU elevation by 12z/Sun, with the profile holding below freezing with only some cooling on Sunday. Trough passage near 00z/Mon in the GFS, but no real cooling, with warming aloft already by early Mon as the upper level ridge builds in. In the current forecast the SAC WFO has snow levels rising to the 4000-5000 ft level as the storm commences on Saturday, and then lowering to 3000-4000 ft on Sunday, along with potential high wind warning criteria winds with the event. Whether all snow or rain to snow, the snow would initially be heavy and wet, getting to somewhat higher ratios on Sunday but certainly not Colorado powder, but the combo of heavy wet snow and strong winds could create major radar issues at BLU by Saturday night, and will be further discussed in the telcon tomorrow. Dave Reynolds mentioned the 4 Jan IOP with its very strong winds and the radar problems that resulted in that event.

Longer range another strong trough approaches (the one west of the Dateline now, which was discussed earlier) but never really makes it into the ARB and instead ridging occurs through next week as this wave drives eventually more to the north into the Pacific Northwest. Other waves then lined up in the Pacific, and the ECMWF 12z run brings the first of these into the area by 1-2 Mar, while the GFS is weaker and farther to the north. Ensembles show quite a bit of uncertainty by ~days 8 through day 15 (~1-8 Mar). So relatively high confidence of a dry period after our big weekend storm for most of next week, then some possibilities as we get into early March.

ed szoke NOAA/CIRA/GSD

February 20, 2008

HMT Forecast Wed 2/20/2008 2000UTC

We've had precip over the ARB since yesterday afternoon and 24 hour totals
ending 16Z vary with Blue Canyon being representative for the mountains
having 0.28". Higher amounts were observed at lower elevations such as 0.52"
at Folsom. This is consistent with the LAPS 24 hr precip accumulation as of
12Z. Over the past few hours the precip has been becoming restricted to the
NERN ARB. Easterly flow aloft as the cutoff slides by to the S is currently
favoring the Reno area. The precip should be spotty over the ARB this afternoon
and precip chances should end by about 01Z Thu according to today's 12Z ESRL
WRF-NMM run.

The PacJet is poised to reach CA on Thu. Waves within the jet will be
amplifying as we get into the weekend. A prominent atmospheric river shows
up from 132W across the Pacific at 00Z Thu. It starts to break up after
that, though we see about 24mm IWV near SFO at 18Z Thu.

The Thu/Fri event is on track as a negatively tilted open wave followed by
a second even more open wave. This double structure has been shown by the GFS
in some of the previous runs. The start time looks a bit earlier at 16Z Thu
continuing until 12Z Sat. There would be a double maximum of precip at about
22Z Thu and 21Z Fri, with about 1.0" total in the GFS. The first burst of
precip should be a bit warmer with 850mb temps of +1C, and the second shot
a bit colder at -0.5C (probably all snow at Blue Canyon). The ECMWF Hi-res has
about the same amount of total precip, while the ESRL WRF-NMM run suggests
favored areas could receive 2" (starting 19Z Thu).

The weekend event has the 500mb low surviving landfall farther south than
we've seen, along the northern CA coast at 18Z Sun. The precip in the GFS
occurs between 21Z Sat and 09Z Mon with a total of 3.0" (similar to yesterday's
12Z run). 850mb temps start out fairly warm at +3C gradually dropping to +0.5C
near the end, thus after much rain some snow is possible at Blue Canyon
towards the end. The ECMWF Hi-res has a deeper low a bit more to the north
and it moves a bit slower suggesting a slightly later end time. However this
model gives less precip with 1.7" over the ARB with the highest amounts a bit
to the south.

The GFS IWV field has about 22mm nosing in over SAC on 00Z Sun with higher
amounts around 30mm upstream to the SW just offshore between Santa Maria
and Monterey. At this point the overall atmospheric river has a scalloped

Longer term it looks like a wave on Wed Feb 27 will ride up over a west coast
ridge and hit mostly in BC. The GFS hints that the PacJet may redevelop in
the WRN Pac around Mar 1 with an associated atmospheric river forming.
This is analogous to what we're seeing this week so we might see some
WX in the ARB around Mar 4.

Steve Albers

February 19, 2008

HMT Forecast Tue 2/19/2008 2020UTC

The PACJet we've been watching for a while is on track to roll into CA in
earnest on Thursday with a precursor wave coming in this afternoon and Wed.
The atmospheric river we've been watching gets into the act beginning late in
the week, at least with pieces breaking off from the main moisture feed.

The first wave undercuts the west coast ridge and gives some precip over the
ARB between 21Z today and 21Z Wed with about .6" in the GFS. The ESRL WRF-NMM
also starts at 21Z today and has similar totals, but ends earlier at 12Z Wed.
The model timing looks good as the LAPS radar analysis shows the main line of
precip just ready to move inland from SFO at 18Z.

The Thu/Fri event comes in as a negatively tilted open wave. The GFS gives
about 1.4" between 21Z Thu and 06Z Sat with the most intense precip about
03Z Fri, slightly better than yesterday's 12Z run. During this storm the 850mb
temps slowly drop from +1.5C to -0.5C so most of the precip at Blue Canyon
should be rain with perhaps some snow towards the end. The ESRL WRF-NMM has
similar timing on the precip start.

The weekend storm has a 500mb low riding the jet and filling in as it
approaches the SW OR coast. The GFS gives about 2.9" between 21Z Sat and
09Z Mon with the bulk of the precip on Sunday. This is a bit more than
yesterday's GFS and a bit slower as the ECMWF was hinting at then. 850mb
temps more gradually drop from +2C to 0C during this event. Over the weekend
the GFS now agrees better with the ECMWF timing as the wave approaches the
coast. The ECMWF has a slightly deeper trough.

We continue to see the long atmospheric river in the 12Z GFS at 06Z Thu
extending westward from 130W back to the Philippines. The details of the
pieces that subsequently break off change somewhat in successive model runs.
There is one piece with about 28mm IWV off the Santa Barbara coast on 06Z Fri.
Then on 00Z Sun we again have about 28mm off Santa Barbara. In comparison the
00Z GFS breaks up the river slightly sooner (after 00Z Thu) with 31mm IWV
reaching Santa Barbara 12Z Fri and 25mm on 00Z Sun Feb 24.

The ECMWF hi-res shows higher 850mb dewpoint values of +6C nosing into Santa
Barbara at 00Z Sun (with a more continuous plume) compared with the ECMWF
low-res and GFS.

Longer term on the GFS we see in interesting atmospheric river possibility
on Mar 3 with a secondary event on Mar 5.

Steve Albers

February 18, 2008

HMT Forecast Mon 2/18/2008 2345UTC

The big picture looks fairly similar to yesterday's forecast. The west coast
ridge will hold to keep things dry for today, though the first weak wave is
approaching as a weak low-latitude cutoff. High clouds from this are already
spreading over the ARB. Some very light precip is possible as this system
swings through between 00Z and 12Z Tue.

Close behind is our Wednesday wave breaking down the ridge further with about
.2" in the GFS from 21Z Tue to 12Z Wed (similar to the GSD WRF-NMM). A 500mb
low phasing in with this wave swings through NRN CA later Wed giving a second
shot of precip with .3" between 12Z and 21Z Wed (this isn't showing up in
the 12Z GSD WRF-NMM forecast).

Next wave in the sequence is for Thu/Fri. The GFS brings about 1.1" between
21Z Thu and 06Z Sat. This open wave (still with a double structure) ushers
in the stronger PacJET and may be a borderline IOP event. Heaviest precip is
during the day Friday as the second component phases in. 850mb temps stay at
about +1C. The ECMWF 12Z run looks fairly similar to the GFS at 12Z Fri.

The atmospheric river in the Pacific reaches its longest length at 00Z Thu
going from 130W westward to the Philippines. After that it breaks up with
pieces of it reaching CA over the next few days. The best piece to reach
CA hits the Santa Barbara area with 32mm IWV on 12Z Sat.

Correspondingly, a bit higher amplitude open wave brings more energy
into Central and SRN CA over the weekend. The ARB is in the northern portion
of the heavy precip area. The GFS gives about 2.5" between 06Z Sat and 21Z Sun
with heavier amounts to the S. Temps start out warm around +3C with fropa
about 06Z Sun dropping the 850mb temp to -3C. The ECMWF is about 6hr slower
and has more of a SW flow that could bring up moisture towards the ARB.
This looks to be the best IOP event for the week.

The long range GFS IWV field suggests the possibility of weak events on Feb 27,
Mar 1, and Mar 4.

Steve Albers

February 17, 2008

HMT Forecast Sun 2/17/2008 2000UTC

West coast ridge remains with us for the next few days. PACJET is set up
westward of 145W and progressing to the east. The low we are looking forward
to presently near 165E is rotating around a more stationary low over the

The progression of short waves is similar to what we saw yesterday. The first
wave coming in remains dry on Tuesday as it undercuts the ridge.

The next wave is on track for Wed Feb 20 with about .25" in the GFS between
00Z and 18Z.

The atmospheric river we've been looking at is still there with important
details changing from day to day. The latest GFS run has the best setup at
18Z Wed reaching from 135W all the way to the Philippines. However it breaks
up as pieces of it reach the CA coast with the ensuing short waves. We get
25-30mm of IWV impinging on the SRN half of CA at 12Z Fri.

The main low we've been tracking looks a bit weaker at 12Z Thu compared with
yesterday's run as it rotates into NW BC. We still have the PACJET with more
of a spread out appearance along the W coast of North America. We then have
short waves reaching NRN CA every 24-36 hours embedded within the jet. The
first one now looks to be between 03Z Thu and 02Z Fri with just about .45" in
the GFS. The ECMWF looks about 6 hours slower with this wave.

There is a clearer demarcation between the two short waves on Thu/Fri compared
with yesterday's GFS run. The later wave looks to be the stronger of the
two coming ashore between 15Z Fri and 08Z Sat with about .8" of precip. So
the sum of the two waves comes out slightly less than in yesterday's run.
Since the flow is fairly zonal the temperatures remain moderate being about
0C at 850mb dropping down to -2C at the end. The ECMWF looks a bit faster
with this second wave.

Next in the progression of waves is from 03Z Sun to 06Z Monday with about
1.35" and is more than in yesterday's run. What remains of the atmospheric
river though is down by LA at 12Z Sun. This system digs more than the Thu/Fri
ones and brings down more cold air with 850mb temps dropping from 0C down
to -4C.

Overall we will want to maintain watching the details of the short waves
and atmospheric river with an IOP possibly commencing between Thu and Sun.

Steve Albers

February 16, 2008

HMT Forecast Sat 2/16/2008 2040UTC

West coast ridge still looks to be in place over the next few days. We also
have a PACJET in the WRN PAC with a 500mb low E of Japan and S of the Kamchatka
Peninsula. This starts to rotate through the jet as this jet moves east
during the GFS model run. Ahead of this the first short wave starting to
break down the ridge comes in on Tue Feb 19, as a fairly dry cutoff almost
sliding in under the ridge in today's 12Z GFS solution.

A second short wave mainly hits WA/OR early Wed Feb 20, with a short period of
very light precip over the Sierra Crest about 00Z Wed. The ECMWF low-res is
about 6-12 hours slower with this wave as it moves inland.

The next stronger system brings a double shot of moisture to the ARB between
about 12Z Thu Feb 21 and 00Z Sat Feb 23, totaling about 1.5" in the GFS.
At 12Z Thu Feb 21 there is a 500mb low (that was east of Japan at the model
initial time) off the BC coast and north of the PACJET. The configuration of
the low and PACJET looks pretty similar between the latest GFS and ECMWF runs.
At the same time there is a rather long atmospheric river in the GFS IWV field
showing up all the way from the Philippines to just offshore of SFO with values
of 34mm IWV. The main feed is shunted to the south by 00Z Fri Feb 22 with some
leftover values of around 28mm near SFO. By 12Z Fri Feb 23 the river has moved
south and broken up considerably. The GFS solution for this system is stronger
compared with yesterday's run.

A followon wave comes in later Sat Feb 23 into Sun Feb 24 with lighter amounts
of precip (around .4").

Steve Albers

February 15, 2008

HMT Forecast Fri 2/15/2008 1950UTC

Looks like same general pattern in GFS model run compared with yesterday,
though some details differ with short waves for mid-late next week. We still
have west coast ridging keeping things high and dry over the next few days.

There is a wave moving north giving precip to BC on Friday. An associated
front could bring some precip down as far south as the WA/OR border on
Friday afternoon.

The initial short wave breaking down the ridge brings some light precip
(still about .2") from 21Z Tue Feb 19 until 09Z Wed Feb 20. The 00Z
ECMWF hi-res run is a bit shallower with this system compared with the 12Z GFS.

The PACJET is better established in the ERN PAC by mid-week (both in the 12Z
GFS and the 00Z ECMWF hi-res runs). Compared with yesterday's GFS deterministic
run the mid-week wave has less of a tropical moisture feed with lighter precip
amounts. The GFS yields just .2" between 06Z Thu Feb 21 and 00Z Fri Feb 22.
Time will tell if this represents a flip-flop in the GFS or a more consistent
trend, so an IOP is still slightly possible as early as Thu.

The following system looks a little more promising at this point with moderate
precip (about .7") coming in Fri Feb 22.

There is an interesting atmospheric river showing up between 140W and 150E
at 00Z Wed Feb 20. As the short waves approach the ARB however we see pieces
of the river being broken off (more than in yesterday's run) to quickly move
southward along the CA coast.

The long term GFS IWV forecast suggests drier conditions with maybe some weak
events around Feb 27 and Mar 1.

Steve Albers

February 14, 2008

HMT Forecast Thu 2/14/2008 2000UTC

Rather sunny and dry over the ARB with a few leftover wave clouds over the
Sierra Crest today. Some light precip was scattered in the ARB the past
24 hours with .12" at Greek Store. The tail end of the trough axis is currently
moving southward in SRN CA, with most precip well inland in the intermountain

There are some waves coming in through the Pacific being diverted northward
by an offshore ridge. There is a decent jet in the WRN Pacific that moves
eastward in the GFS during next week. This should gradually break down the
offshore ridge as we enter the mid-late next week time frame.

From today (Feb 14) to Feb 16 we have a wave going north bringing precip into
BC. The next system starts to break down the ridge with light-moderate precip
(.2" in the GFS) arriving in the ARB Tue Feb 19.

Following that a reasonable looking PACJET directs precip into the ARB with
heaviest amounts late Wed Feb 20 into Thu Feb 21. The GFS yields some healthy
precip amounts totaling about 2.8" as lighter amounts trail off into Friday.
This shows an atmospheric river signature briefly though it starts to break
up as it approaches CA. Nonetheless the GFS forecast is for 35-40mm of IWV
reaching SFO for 00Z Thu Feb 21. We'll want to watch to see if this holds
enough in future model runs to suggest calling an IOP.

Time will tell whether we will get a long term pattern change to more zonal
flow or if the west coast ridge returns after next week.

Steve Albers

February 13, 2008

HMT wx forecast discussion: 13 February 2008 at 1945 UTC

A NE-SW band of enhanced cold cloud tops in the IR satellite imagery is clearly evident diving SSEward across NV and SEern CA this morning in association with the now sharply digging S/W trough. The Reno 88D radar shows a precip band beneath the IR cloud enhancement, although the echoes are remaining mostly E of the Sierra Crest. The Sacramento 88D has tagged only a few scant echoes near the Crest midmorning. As the upward-motion forcing with the S/W quickly propagates into the Great Basin, the ARB will transition into strong subsidence this afternoon in a post-cold-frontal environment. Hence, what little precip may fall across the ARB today will be minimal at best and only short-lived.

As the S/W circulation intensifies over the Great Basin later today into tomorrow, brisk N to NE winds will blow on its back side across the ARB, partly due to a developing large-scale pressure gradient of significant magnitude. W should also expect a period of downslope mountain-wave dynamics tonight into tomorrow, which should further intensify the NEerlies along the western slope of the Sierra. No precip will fall tonight or tomorrow, although temperatures will be cooler than the last couple of days.

Temperatures will quickly rebound by Friday and the weekend, as high-amplitude ridging aloft will once again take hold across CA. It will remain dry. Thereafter, the picture has not changed appreciably for the long range. S/W troughs propagate into the diffluent upper-level flow across the EPAC, weaken, and make landfall in BC, the PacNW, or perhaps as far S as N CA through the middle of next week (~20 Feb). These systems will likely not yield much (or any) precip for the ARB. Thereafter, perhaps zonal wave energy will finally reach CA to provide moisture to the ARB. We shall see.

Paul Neiman

February 12, 2008

HMT wx forecast discussion: 12 February 2008 at 1945 UTC

Same outlook as yesterday: high and dry for the foreseeable future.

The latest satellite imagery clearly shows a prominent midlevel S/W trough riding over the EPAC ridge and making landfall across BC and the WA coast. A desiccated, thin comma-cloud tail extends SSWward from near the circulation center to a position just offshore of OR and extreme N CA before dying out. Meanwhile, the ARB is solidly in subsidence and is expecting another sunny and mild day today.

The newest model solutions still show the S/W digging vigorously SSEward into the Great Basin on Wednesday, as strong cold advection and a developing 300-mb jet become established on the back side of the wave. QPF is minimal at best on Wednesday across the ARB as the tail end of the midtrop vorticity max scrapes the Sierra Crest. All in all, the strong large-scale dynamics and enhanced moisture will remain E of our area. Perhaps the most noteworthy aspect of this storm for the ARB will be the brisk (and dry) N to NE winds that blow on the back side of the wave later Wednesday and Wednesday night, due partly to a developing large-scale pressure gradient of significant magnitude. Downslope mountain-wave dynamics may further intensify the NEerlies on Wednesday night.

The EPAC ridge amplifies in a big way on Thursday and beyond, so the dry wx shall continue unabated. Subsequent S/W troughs propagate into the diffluent upper-level flow across the EPAC, weaken, and make landfall in BC, the PacNW, or perhaps as far S as N CA through the middle of next week (~20 Feb). These systems will likely not yield much precip for the ARB. Thereafter, perhaps some zonal wave energy will finally reach CA.

Paul Neiman

February 11, 2008

HMT wx forecast discussion: 11 February 2008 at 1945 UTC

And the beat goes on… high and dry for the foreseeable future.

Deep tropospheric ridging today and tomorrow across the ARB will provide mostly clear and mild conditions. Meanwhile, a prominent midlevel S/W trough is evident in the satellite loops, presently crossing the 150W meridian at ~44N latitude. This wave is moving ENE and will flatten the EPAC ridge on Tuesday and make landfall in the PacNW Tuesday evening.

Once E of the ridge axis, the S/W will vigorously dig SSEward into the Great Basin on Wednesday, as strong cold advection and a developing 300-mb jet become established on the back side of the wave. The newest model solutions continue the decreasing trend for QPF in the ARB during this event. Specifically, the strong large-scale dynamics and enhanced moisture remain N-E of our area. Also, as was the case in previous model solutions, the low- to mid-level flow remains NWerly prior to, during, and after the wave passage, which is quite unfavorable for orographic forcing. Hence, expect little or no precip across the ARB with the wave passage.

The EPAC ridge amplifies yet again Thursday and beyond, so the dry wx shall continue. Subsequent S/W troughs propagate into the diffluent upper-level flow across the EPAC, weaken, and make landfall in the PacNW and/or BC through the middle of next week (~20 Feb). Thereafter, perhaps some zonal wave energy will finally reach CA… maybe.

Paul Neiman

February 10, 2008

HMT Forecast Discussion made at 1945z Sunday 10 Feb 2008

Outlook little changed from yesterday ...

Weak wave passing over northern CA at the moment has only bands of Cirrus with it.

The stronger wave for Tu-W 12-13 Feb is presently associated with a deep surface low S of Cold Bay AK, and a prominent upper-tropospheric short-wave trough feature in water-vapor imagery near 35N/170W. Progs all indicate this latter feature moving ENE through the mean ridge position offshore and then splitting as it approaches the coast of BC and the Northwest, with the southern portion heading southeastward over the ARB late Tu and W, cutting off over AZ by early Th 14 Feb, much as was suggested yesterday. The outlook for pcpn over the ARB with this is no better than it was yda--the best one can hope for is nil to an inch or 2 of dry snow over the higher ridges of the Sierra. The dividing streamline (between coast-parallel southerly flow to the N and northerly flow to the S) along the W coast is forecast to remain near or N of the OR-CA border as this weather system approaches the coast Tu, and consequently little or no pcpn can be expected, even along the north coast.

Farther ahead, the outlook remains grim for more pcpn. (My cousin who farms a section near Clements on the Mokelumne River reports that the ground is saturated at his place.) The GFS ensembles frm 00Z 10 Feb indicate with unanimity that the ARB will remain dry through 12Z Tu 19 Feb. This morning's 12z GRS ensembles extend this streak to 12Z W 20 Feb! Beyond this, indications are of overall larger amplitude and the possibility of more pcpn in northern CA, but there is no clear consensus for a significant event, just indication that the probabilities increase to climatological for the date by late next week.

John B.

February 9, 2008

HMT Wx Forecast made 1945z 9 Feb 08

Outlook for the next several days is dry. The current pattern of ridging in the eastern Pacific and mean upper trough position over the CONUS is expected to hold for the next week at least. It is worth noting that the NCEP global ensemble members, visible on the Penn State Ewall page, are amazingly dry out through 10 days. Only 2 of the 12 indicate any pcpn over the American RIver Basin for the 10-d period ending 1200 UTC 19 Feb, and both of these show < 0.10" total for that entire 10-d period. The ECMWF and GFS hi res runs are more similar than usual, and the EC is also showing ZERO accumulated pcpn over the ARB for the same 10-d period.

Two short-wave troughs will pass over the area in the next few days. The first of these is approaching the Northwest coast at present and giving some light pcpn in western WA and nw OR. I doubt any pcpn will fall anywhere in CA out of this weakening feature, which is now within mid-level larger scale difluence. A second, stronger storm now in the central Pacific will approach the coast Tu 12 Feb. The ECMWF and GFS are indicating the usual weakening of the surface low as it approaches the coast. The upper level portion of this feature will split as it enters the mean difluence near the coast, with the southern portion heading southeastward to form a cutoff low over srn CA/AZ/Sonora. As this feature passes over the area late Tu 12 Feb on its way southeastward, there exists the possibility of a few inches of orographic snow over the higher ridges of the Sierra. However, mosture will be limited and the flow not favorable for terrain forced lift (700mb flow may not get out of the NW quadrant), so I would be surprised to see amounts greater than 1/4" to 1/3" at best, and this only if this feature digs farther W than forecast.

Beyond this, the outlook is poor for the last half of next week. By the weekend, the global models are indicating that the present strong jet coming off the Asian coast and across the western Pacific will extend farther east. (At present, and for the last several days the strong jet has terminated in an upper difluent region between HI and AK, with mean low-latitude troughing SE of HI and higher latitude ridging over the eastern Pacific.) This may eventually pave the way for a shifting inland of the ridge now over the eastern Pacific and for a greater potential for IOPs the week of Presidents' Day.

John B.

February 8, 2008

HMT wx forecast discussion: 8 February 2008 at 1945 UTC

Dry and mild conditions will prevail today and tomorrow across the Sierra Nevada, as midtrop ridging just offshore of CA provides dry, subsident W to NWerly flow aloft. The ridge will flatten later tomorrow and Sunday, as S/W cyclonic vorticity spokes take aim at the PacNW. The leading S/W is presently visible in the satellite imagery moving Eward across ~155W between 40-50N. The ARB will remain well S of any dynamics and precipitation associated with these transient disturbances. Hence, expect the dry wx to persist on Sunday.

Building geopotential heights at midlevels will once again occur offshore of the West Coast early next week, so do not expect any wet/white wx in the Sierra through Tuesday February 12th. Thereafter, the next in the series of S/W troughs is progged to come ashore Tuesday night and Wednesday. Unlike yesterday’s GFS model runs that kept this S/W well N of the ARB, last night’s solutions dig this wave into the Great Basin, although with no significant precip in the ARB. The EC model digs this wave with more vigor in the NWerly flow along the West Coast (as in yesterday’s EC runs), with the possibility of precip and low snow levels in the ARB. Given the history of this winter to date, I believe the EC solution is a distinct possibility. However, it is questionable whether either scenario will yield conditions favorable for an IOP.

Another cold, digging wave may be in store for the ARB toward the end of next week.

Paul Neiman

February 7, 2008

HMT wx forecast discussion: 7 February 2008 at 1945 UTC

All is quiet on the western front. The latest SSM/I and AMSU satellite imagery of integrated water vapor shows very low values (<1.5 cm) across the entire eastern Pacific basin, save a dying band of slightly enhanced PW (1.5 to near 2.0 cm) impacting the PacNW associated with the shortwave trough embedded in the racetrack northwesterly flow aloft. Companion IR and upper tropospheric vapor-channel loops have a firm lock on this shortwave and its somewhat enhanced cloudiness in WA, OR, and northernmost CA. Some light shower activity with this S/W has crossed the Oregon border into extreme N CA, but the precip will remain well N of the ARB as the S/W propagates SEward into the Great Basin today.

Ridge building aloft will commence just offshore of CA tonight and tomorrow, thus keeping the ARB in dry, subsident, NWerly flow aloft. The ridge flattens and moves inland on Saturday into Sunday, as weak S/W energy begins coming ashore into the PacNW. The 12Z NAM is farther S and a bit stronger than its 12Z GFS counterpart with the landfall of the next S/W later Sunday into Monday centered near 00Z 11. In fact, the 12Z NAM shows a 500 mb vort. max approaching the OR/CA coast at 00Z 11. Nevertheless, even with this southern-track scenario, moisture should be limited with this system… especially on its southern flank across the ARB, and the lower-level W to NW flow will be unfavorable for orographic forcing. In addition, as with the majority of our other storms this winter, temperatures will be relatively cool with this transient wave. Hence, do not expect much in the way of meaningful precip.

As this next wave drops into the Great Basin early next week, midtrop heights will once again rebound in response to ridge building in the eastern Pacific. Based on the extended model forecasts (both the control and ensemble solutions), the ridge will flatten every couple days thereafter during the landfall of transient S/W troughs along the West Coast. The GFS keeps these waves mostly N of the ARB with no significant precip., whereas the new 12Z EC model tends to dig these waves in the NWerly flow aloft (downstream of the EPAC ridge) along the West Coast, with the possibility of precip and low snow levels in the ARB. Given the history of this winter to date, I cannot discount this latter scenario from playing out. Time will tell.

Paul Neiman

February 6, 2008

HMT Forecast 2/6/2008 19:41 UTC

Not much has changed since yesterday over the ARB. The previous forecasts still remain valid. This morning the satellite water vapor imagery indicated a plume of moisture reaching the E Pacific coastal areas near 49N 127W. The near term models show coastal showers a possibility today/tomorrow with the main moisture going north of the ARB. However, I would not rule out a brief shower or two.

Models show a ridge building over the coast that reaches its maximum on or about 06UT 2/9 (Saturday) and then a short wave crossing the area on or about 12UT 2/10. It is not anticipated that this will bring significant precipitation to the ARB. Following this, the ridge rebounds by 00UT 2/12 and a cut-off 568 low develops beneath the ridge at 29N 119W. By 18UT 2/13 the cutoff opens and progresses as an open wave to the south of the region (this is in the GFS). In contrast, the EC doesn't create the intense cutoff low under the ridge but has a more vigorous open wave progressing over the ridge that brings 0.02 inches of precip to the ARB on or about 2/13.

During yesterday's telecon the question arose regarding the conditions for melting the snowpack that exists in the ARB. I am not an expert in this area but did run some numbers. The current integrated daily solar energy flux for today over the ARB is 9.87 MJ/m^2 (horizontal plane), this for comparison purposes is about 50% more energy than the area was receiving at winter solstice (6.42 MJ/m^2), and for further comparison the region receives 27.37 MJ/m^2 at summer solstice. Also, I forecast today's high temperature for the ARB to be about 44F, cooler at mountain tops. Unfortunately, I lack the background knowledge to translate this into snowmelt from sensible and radiative heat flux, but maybe someone reading this blog can determine that from these numbers.

The general forecast for near term IOP events remains nil. The long range forecast past 228 hours looks to break down rapidly in the ensembles. Up to that time, there does not appear to be an IOP opportunity. Perhaps we can enlist climate experts to help us with a long-term forecast discussion.

Dan Birkenheuer ESRL/GSD

February 5, 2008

HMT Forecast 2/5/2008 19:35 UTC

Yesterday's forecast can be repeated today with confidence. Today we see 2 closed circulations in the Pacific, one at 52N 132W and the other further west at 47N 172W. Between them is probably the most moisture over the mid latitudes upstream of the CONUS at 43N 161W. Unfortunately, the near term forecasts do not show much hope in bringing this moisture to the ARB.

Last night's 06UT GFS run looked a lot like the 00UT EC run with minor precipitation entering the ARB (0.5 inches) on or near 2/13, the date we have been mentioning in discussions for the past several days. However, this morning's 12UT GFS run is now looking even more pessimistic for any chance of precipitation in the ARB on that date. The upper air pattern in the latest GFS run appears to stay more zonal and the cutoffs in earlier runs now appear as open progressive waves that traverse the CONUS rapidly. This mornings GFS basically has the system going even further north than prior runs and leaving the ARB dry on the 13th. Anything really beyond this point in time is highly speculative, since after 216 hours, the GFS ensembles produce some forecasts with precipitation; most of the ensemble members remain dry. The ensemble spaghetti plots also diverge rapidly after 216 hours leading me to think that we can't really say much beyond this timeframe.

As of this morning, the forecast for the ARB is to remain dry for at least the next 216 hours with possibly showers at times well under IOP threshold criteria.

Dan Birkenheuer ESRL/GSD

February 4, 2008

HMT Forecast 2/4/2008 19:39 UTC

The current forecast problem is now the long range prospect of future precipitation events. This morning's water vapor satellite imagery shows a strong ridge building at 51N 142W and a closed circulation at 40N 166E. The ridge is forecast to progress over the CONUS this week basically suppressing short waves and potential moist surges.

The next possible plume of moisture has been forecast by past models and again appears to be associated with a short wave coming through the ridge by Wednesday 2/6. The SREF gives a very modest 35% probability of precipitation between 12-18UT 2/6. At the same time the GFS shows 0 precip (system brings precipitation to the north of the ARB only), and the NAM produces only a trace.

After this system passes, zonal flow dominates for some time. There is indication of some slight ridging in the pattern by Saturday 2/9. Over the weekend, the GFS (2/4 00ut run) deterministic model shows a possible 2/10 event bringing 1.17+ inches to the ARB, but the EC model does not have anything close to an event at this time. Therefore, I discount the possibility. However, all models do have an event on or about Wednesday 2/13. Looking at past forecasts, this date has been mentioned consistently since last week as a possible timeframe for something. The EC model starts precipitation over the ARB at 00UT on this date and has it continuing until the end of the run (at least until Thursday) with 1.82+ inches of precipitation. Since all models are showing something in this timeframe I will go with this as the next likely possibility of something in the ARB and will continue to monitor the potential the remainder of this week.

Dan Birkenheuer ESRL/GSD

February 3, 2008

HMT Forecast 2/3/2008 19:40 UTC

The current event is winding down and may end at 18UT this morning with the NAM forecast trof passage or it may have already ended according to satellite imagery. Water vapor imagery indicates that drier air was entrained over the ARB at about 09UT and may coincide with early trof passage. NAM still puts down only 0.17 inches and GFS only 0.05 inches after 12UT. Models did reasonable job considering the widespread precipitation amounts in the past 24 hours ending 8am Pacific time this morning, BLU reported 2.87 inches, Huysink 2.44, Auburn 1.00, Georgetown 1.84, and Forni Ridge 1.28 inches. NAM continues to show this to be overall a warmer event than initially forecast with freezing levels near 875 hPa. Satellite water vapor imagery has a building ridge at 46N 153W and it is amplifying driving moisture surges more N-S with time as waves traverse the east side of the ridge.

Models consistent with next moisture surge near 12UT 2/6 but it sweeps by quickly, GFS only produces 0.09 inches in ARB with this.

Next moisture opportunity is near 00UT 2/8 and GFS barely registers precip in the ARB.

Long range forecast appears to be dry after that with maybe an event near 2/13.

Dan Birkenheuer ESRL/GSD

February 2, 2008

HMT Forecast 2/2/2008 19:44 UTC

This morning (16UT) the ARB is already seeing some signs of early precipitation with BLU reporting 23F and S-; stations to the north have light precip while stations mostly in and south of the ARB are still reporting partly cloudy conditions and no precipitation.

The satellite water vapor imagery shows a good N-S oriented moisture plume west of HI extending from 24N 173E to 47N 174W, there is also evidence of a low spinning up at 25N 164 W.

Yesterday the decision was made not to conduct an IOP for the system progged to start in the ARB this evening. Today's models have this system producing about the same amount of precipitation as in prior forecasts with this morning's NAM showing 2.4 inches consistent with earlier forecasts. Winds with this system also seem to be about the same but maybe a tad lower in intensity with 30 kts at low elevations and 50kts at higher elevations from the SW at 12UT 2/3 with the duration of the stronger winds less that prior forecasts indicated. This timeframe coincides with the GFS model's moisture plume impacting the area at its peak; furthermore the moisture plume appears to be stronger, at least out off the coast with TPW values today reaching 3cm. Precip at this time (12-15ut) is shown to be about 0.8 inches. The one thing that is different about this event in this morning's models is that it appears to be a bit warmer than prior forecasts showed with freezing level starting around 3500ft and then rising to 5000 ft at 12UT 2/3, about the same time of greatest windspeed and just before the trof passes at 18UT 2/3. The SREF ensembles show the event to end at 03UT 2/4 and this is consistent with earlier model runs.

Following this event the GFS shows a 1.6 cm TPW plume (small in size) sweeping by the ARB at 06UT 2/6, again consistent with prior forecasts.

In addition, today we see some "new" potential events in the GFS, one at 18UT 2/8 and the one we have seen for some time for Wednesday 2/13. Studying the upper level wave patterns it appears that as we move through this week the high pressure ridge off the Pacific coast retrogrades slightly so that moisture plumes are driven north to south on or near the ARB instead of a preferred W to E trajectory. So if "rivers" (I hesitate to call them this since they have no direct trajectory out of the tropics) make it into the area they move out swiftly and there appears to be a greater chance that they do not make it far enough inland, reducing any impact on the ARB.

Dan Birkenheuer ESRL/GSD

February 1, 2008

HMT Forecast 2/1/2008 19:42 UTC

This morning we are seeing the tapering off of the event that began yesterday morning. As of 7am Pacific time, BLU had reported 1.72 inches, Huysink 1.92, Auburn 1.12, and Georgetown 1.04 showing a trend of higher totals in the north of the ARB over the south. This morning's NAM has the potential for additional accumulations of up to 0.4 inches with the event ending sometime around 15ut morning according the SREF ensembles, so I think the NAM might be over doing the precip on this. These moisture amounts were right in line with the HMT high res ensemble runs yesterday that showed the NAM to maybe be a bit high on the total precip side as well, and gives further credibility to the HMT project's high resolution ensemble forecasts.

This morning we have another moisture surge in the IR water vapor imagery nosing its way toward the coast located at about 44N 139W. This is the source for the next system that is still on track to start another precipitation event between 21UT 2/2 and 00UT 2/3. Today this event still looks to be fairly cold in overall character with freezing levels starting near 950 hPa (1960 ft), rising quickly to 920 hPa (2900ft) and then further lifting (gradually) to 900 hPa (3500ft) by 15UT 2/3 where it remains until the end of the event. The moisture plume dimensions as shown by GFS are broader (N-S) in nature over the current system with a TPW content of about 2.5cm and therefore should result in a longer lasting event for the ARB. This is corroborated by the appearance of the plume in this morning's satellite imagery, but the plume may change dimensions between now and Saturday evening, even so, it looks like the GFS is doing a good job on describing the areal extent of the moisture. As noted yesterday, the winds associated with this event remain stronger and more favorable than the current event with winds beginning at SSW at 30kts, increasing to SW at 50kts for a considerable timeframe (06UT - 15UT) 2/3 after which they drop at 15UT coinciding with a trof passage and the likely start of the end of the event with the winds going westerly to 30 and finally northerly at 15 near 00UT 2/4. The NAM produces 2.83 inches of liquid equivalent, while last night's GFS had 2.14 and this morning's GFS produced 2.25 inches. So the models are all pretty consistent with 2+ inches of precipitation. By all accounts this could be considered a favorable IOP candidate with the exception of the cold nature of the event with the freezing level staying at relatively low levels throughout.

Following this event there appear to be some minor precipitation opportunities, but nothing to get excited about yet. The 06UT 2/6 event is both weaker and faster moving than the "Saturday evening" event mentioned in the prior paragraph. Following this there appears to be another potential plume of moisture entering the vicinity of the ARB, on or about Wednesday 2/13.

Therefore, as things currently stand, after this next potential opportunity (Saturday evening local time) the ARB enters an extended period of minor to inconsequential events and possibly an extended dry period.

Dan Birkenheuer ESRL/GSD